1. Field of the Invention
The present invention relates to a porous ferrite core material for an electrophotographic developer used in a two-component electrophotographic developer used in copiers, printers and the like, a resin-coated ferrite carrier and an electrophotographic developer using the resin-coated ferrite carrier.
2. Description of the Related Art
An electrophotographic development method conducts development by adhering toner particles in a developer to an electrostatic latent image formed on a photoreceptor. The developers used in such a method are classified into two-component developers composed of toner particles and carrier particles and one-component developers using only toner particles.
As the development methods using two-component developers composed of toner particles and carrier particles among such developers, a cascade method and the like have long been adopted; currently, however, magnetic brush methods using a magnet roll are predominant.
In a two-component developer, the carrier particles serve as a carrying substance to form a toner image on the photoreceptor in such a way that the carrier particles are stirred together with the toner particles in a developer box filled with the developer to impart a desired charge to the toner particles, and further, convey the thus charged toner particles to the surface of the photoreceptor to form the toner image on the photoreceptor. The carrier particles remaining on a development roll which holds a magnet again return from the development roll to the developer box to be mixed and stirred with the fresh toner particles to be repeatedly used for a predetermined period of time.
In contrast to a one-component developer, a two-component developer is such that the carrier particles are mixed and stirred with the toner particles, thus charge the toner particles, and further have a function to convey the toner particles, and a two-component developer is excellent in the controllability in designing developers. Accordingly, two-component developers are suitable for apparatuses such as full-color development apparatuses required to offer high image quality and high-speed printing apparatuses required to be satisfactory in the reliability and durability in image maintenance.
In two-component developers used in the above-described manner, the image properties such as the image density, fogging, white spot, gradation and resolution are each required to exhibit a predetermined value from the initial stage, and further these properties are required to be invariant and to be stably maintained during the endurance printing. For the purpose of stably maintaining these properties, the properties of the carrier particles contained in the two-component developers are required to be stable.
As the carrier particles which form two-component developers, there have hitherto been used various carriers such as iron powder carriers, ferrite carriers, resin-coated ferrite carriers and magnetic powder-dispersed resin carriers.
Recently office networking has been promoted, and the age of monofunctional copiers develops into the age of multifunctional copiers; the service system has also shifted from the age of the system such that a contracted service man conducts periodic maintenance inclusive of the replacement of the developer to the age of the maintenance-free system; thus, the market has further enhanced demand for further longer operating life of the developer.
Under such circumstances, for the purpose of reducing the carrier particle weight and extending the developer operating life, Japanese Patent Laid-Open No. 5-40367 and the like have proposed a variety of magnetic powder-dispersed carriers in each of which magnetic fine particles are dispersed in a resin.
Such magnetic powder-dispersed carriers can be reduced in true density by decreasing the amounts of the magnetic fine particles and can be alleviated in stress caused by stirring, and hence can be prevented from the abrasion and exfoliation of the coating film and accordingly can offer stable image properties over a long period of time.
However, the magnetic powder-dispersed carrier is high in carrier resistance because the magnetic fine particles are covered with a binder resin. Consequently, the magnetic powder-dispersed carrier offers a problem that a sufficient image density is hardly obtained.
The magnetic powder-dispersed carrier is prepared by agglomerating magnetic fine particles with a binder resin, and hence offers, as the case may be, a problem that the magnetic fine particles are detached due to the stirring stress or the impact in the developing device or a problem that the carrier particles themselves are cracked probably because the magnetic powder-dispersed carriers are inferior in mechanical strength to the iron powder carriers and ferrite carriers having hitherto been used. The detached magnetic fine particles and the cracked carrier particles adhere to the photoreceptor to cause image defects as the case may be.
Additionally, although the magnetic powder-dispersed carriers can be produced by two methods, namely, a pulverizing method and a polymerizing method, the pulverizing method is poor in yield, and the polymerizing method involves complicated production steps, and hence both methods suffer from a problem that the production cost is high.
As a substitute for the magnetic powder-dispersed carrier, there has been proposed a large number of resin-filled carriers in which the voids in a porous carrier core material are filled with a resin. For example, Japanese Patent Laid-Open No. 2006-337579 proposes a resin-filled carrier prepared by filling a resin in a ferrite core material having a porosity of 10 to 60%, Japanese Patent Laid-Open No. 2006-337579 discloses that: various methods are usable as the method for filling a resin in a core material for a resin-filled carrier; examples of such a method include a dry method, a spray drying method based on a fluidized bed, a rotary drying method and a dip-and-dry method using a universal stirrer or the like; and these methods are appropriately selected according to the core material and the resin to be used.
Further, Japanese Patent Laid-Open No. 2007-133100 describes a carrier obtained by impregnating a resin into a porous magnetic material and a carrier obtained by coating the surface of a core material with a large amount of a resin. It is stated that the true specific gravities of these carriers are light, and hence by using these carriers in a refill developer for a two-component development method in which development is conducted while a refill developer having a toner and a carrier is being fed to a developing device, with a superfluous fraction of the carrier in the developing device being discharged, where necessary, from the developing device, the superfluous fraction of the carrier can be smoothly discharged together with the toner.
The porous magnetic powers described in these Japanese Patent Laid-Open Nos. 2006-337579 and 2007-133100 intend to achieve various properties required for ferrite carriers solely by impregnating resins into porous core materials. However, such impregnation of resins into the porous core materials takes time, and additionally, when expensive resins such as a silicone resin, a fluororesin or a fluorine-modified silicone resin is used, the resulting carrier becomes inevitably expensive. Accordingly, it is difficult to claim that resin-impregnated ferrite carriers will become widely used.
Japanese Patent Laid-Open No. 2009-244572 describes a carrier core material for an electrophotographic developer wherein the carrier core material includes 3 to 100% by number of hollow particles having an iron content of 36 to 78% by weight, a carrier for an electrophotographic developer prepared by coating with a resin the surface of the carrier core material, and the production methods of these.
However, in Japanese Patent Laid-Open No. 2009-244572, as a core material obtained by thermal spraying sintering, the apparent density of the core material is small; however, the production method is limited to thermal spraying sintering, hence it is impossible to further reduce the apparent density, and consequently, a long operating life of the developer cannot be achieved.
Japanese Patent Laid-Open No. 2009-175666 discloses a resin-filled carrier for an electrophotographic developer using a porous ferrite core material having a pore volume of 0.055 to 0.16 ml/g and a peak pore size of 0.2 to 0.7 μm.
The porous ferrite core material described in Japanese Patent Laid-Open No. 2009-175666 has a high dielectric breakdown voltage and also has a high fracture strength of particles, but has a low apparent density and does not impede the impregnation of a resin.
Japanese Patent Republication No. 2005-062132 describes a resin-coated carrier for an electrophotographic developer including a spherical ferrite particle specified in volume average particle size, degree of surface uniformity, average sphericity ratio and sphericity ratio standard deviation, a production method thereof, and an electrophotographic developer.
However, as is clear from Examples and Comparative Examples of Japanese Patent Republication No. 2005-062132, even a sintering performed at a high sintering temperature with a rotary kiln does not reduce the apparent density and cannot attain a long operating life as a developer.
As shown by these conventional techniques, there has never been obtained a ferrite core material particle having a low apparent density like a resin-filled ferrite carrier, making it extremely difficult for the impregnation of a resin to occur and having small asperities on the surface of the particle. Additionally, there have never been obtained a carrier for an electrophotographic developer using such a core material particle and a developer using the carrier.